Field of the Invention
The present disclosure relates to a development device and an image forming apparatus, and more particularly to a development device and an image forming apparatus for forming an image by electrophotography.
Description of the Related Art
Electrophotographic image forming apparatuses are widely used. An electrophotographic image forming apparatus includes a development device. The development device supplies developer to the photoconductor to develop an electrostatic latent image formed on the photoconductor.
Referring to FIG. 10, a development device 50X included in an image forming apparatus will be described. FIG. 10 is a diagram showing the internal structure of development device 50X. As shown in FIG. 10, development device 50X includes a restriction member 56 for regulating the amount of developer conveyed and a developer carrier 60 for carrying the developer. Developer carrier 60 includes a fixed magnet roller 52 and a sleeve 53 provided rotatably on the surface of magnet roller 52 to convey the developer downstream in the rotation direction.
The developer is composed of toner and carrier. Toner and carrier are stirred in the inside of development device 50X to produce static electricity. The charged developer is attracted to magnet roller 52 and adheres to sleeve 53. Developer carrier 60 allows sleeve 53 to rotate to convey the developer adhering to sleeve 53 to restriction member 56. The developer is leveled off when passing through restriction member 56. The amount of developer conveyed thus becomes uniform.
The amount of developer conveyed may vary with various factors. If the amount of developer conveyed varies, the print quality is degraded. It is therefore desired to equalize the amount of developer conveyed. In connection with the technique for suppressing variation of the amount of developer conveyed, for example, Japanese Laid-Open Patent Publication No. 2013-200547 discloses a development device that “suppresses variation of the amount of developer conveyed over time and stabilizes the image density”.
The developer adhering to the surface of sleeve 53 is affected by the magnetic force by magnet roller 52. In FIG. 10, the magnitude of magnetic force received by the developer from magnet roller 52 is depicted as magnetic line of force X. The developer receiving magnetic force from magnet roller 52 is in a continuous state in the direction of magnetic force on sleeve 53. The conveyed developer then comes into contact with restriction member 56 and is leveled off by restriction member 56.
When the magnetic force exerted on the developer in the vicinity of restriction member 56 is too large, excessive force is exerted on the developer and degrades the developer. On the other hand, when the magnetic force exerted on the developer in the vicinity of restriction member 56 is too small, development device 50X fails to stabilize the amount of developer conveyed. It is therefore important to stabilize the magnetic force exerted on the developer in the vicinity of restriction member 56.
The development device disclosed in Japanese Laid-Open Patent Publication No. 2013-200547 forms symmetrical magnetic line of force X on sleeve 53. Therefore, the inclination of magnetic line of force X in the vicinity of restriction member 56 increases. As a result, slight displacement of magnet roller 52 causes variation of magnetic force in restriction member 56 more than expected. The development device disclosed in Japanese Laid-Open Patent Publication No. 2013-200547 therefore fails to stabilize the amount of developer conveyed.